Either matters. You’re talking about things like IPC (instructions per clock) versus actual clockrate. Broadly speaking, if your IPC is 10% higher, you can clock 10% lower and achieve the same performance. However, IPC is fixed in the hardware, clockrate depends to some degree on other factors such as yield. Not all CPUs in a given batch will run stable at the same clockrate, that’s why they are binned and sold as different products, even though they are essentially the same chips. Intel has had their 14nm process running for quite a while and their clockrates did go up with every revision. AMD is just now starting with 14nm CPUs.
AMD has demonstrated with this test that their Zen chips are comparable in IPC with Intel’s CPUs, by running both chips at 3Ghz. However, that doesn’t answer the question at which clockrates the chip can really run. A 4Ghz i7 core will still outperform a 3Ghz Zen core by about 30% (ignoring memory here). This could translate into 30% more FPS in a CPU-bound game, for instance. The Intel chip they have tested against runs at least at 3.2Ghz, so why not clock the Zen chip at 3.2Ghz as well? The answer might be, 3.2Ghz isn’t fully stable right now (with 8 cores).
Now, for the quadcore to be truly competitive with Intel in terms of performance, it needs to run at 4Ghz+. Otherwise, it needs to be priced very aggressively and aggressive pricing isn’t bringing great profits.
If the architecture is good enough and optimized enough, I can see Zen outperforming in single-core computing despite the clock being lower.
Everything known so far points to it being about on par, which is already great feat considering the difference in R&D resources between AMD and Intel.